Ultrasonic electroacoustic transducer

ABSTRACT

An ultrasonic transducer has a pair of transducer elements (30,31), polarised in opposite directions, which are mounted between, and in intimate contact with, respective front face electrodes (32) and back face electrodes (33). The front face electrodes are each earthed. The back face electrodes are each connected to a respective input/output terminal (34,35). The input/output terminals are supplied with activating pulses of opposite polarity, produced using a differential pulse generator (39) or a transformer arrangement (43), when the transducer is operating in the transmit mode. When the transducer is operating in the receive mode, pulses of opposite polarity are generated at the back face electrodes when an ultrasonic pressure wave is incident upon the front face electrodes. These pulses are differentially summed using a differential amplifier (40) or a transformer arrangement (43). Impedance matching is preferably used to optimise performance. Such a transducer has a substantially reduced pick-up of environmental noise and thus has an improved signal to noise ratio when in use.

TECHNICAL FIELD

This invention concerns ultrasonic transducers. More particularly, itconcerns ultrasonic transducers which are significantly less susceptiblethan conventional ultrasonic transducers to the adverse influence ofenvironmental electrical noise.

BACKGROUND TO THE INVENTION

Ultrasonic transducers which, when operating in the transmit mode,convert electrical pulses into sound waves at ultrasonic frequencies(and which, when operating in the receive mode, generate electricalsignals when a sound wave at an ultrasonic frequency is incident uponthem) are well known. They are used in a variety of ultrasound echoranging applications, including in ultrasound scanning equipment usedfor medical diagnosis and in non-destructive testing.

The conventional ultrasonic transducer comprises a single transducerelement sandwiched between front and back electrodes. The element may bea polarised ferro-electric ceramic, or a polarised polymer piezoelectriccomposite ceramic and/or polymer material. An alternative known form oftransducer, the dual back-face electrode transducer, comprises a pair oftransducer elements poled in opposite directions, with connections onlyto the two back electrodes of the transducer.

A problem that has existed for a considerable time is that an ultrasonictransducer is susceptible to electrical noise in the environment inwhich the transducer is used. This is due to the unbalanced nature ofthe driving circuit, and is accentuated by a high output impedance ofthe transducer. Even when shielded (coaxial) cables are used with thetransducer, there is a significant tendency for noise pick-up. Thusconsiderable care (and expense) is required to maintain a high signal tonoise ratio in conditions where environmental electrical noise may swampthe ultrasonic signals that are generated or received by the transducer(particularly when the transducer is operating in the receive mode).

One technique for reducing noise in transducers--particularly directedto microphone electroacoustical transducers which include elements madefrom barium titanate or PZT (PbZrO₃.PbTiO₃)--is described in thespecification of U.S. Pat. No. 4,751,418. That technique involves theuse of a reference voltage source (which may be earth potential)connected to the input line from the transducer to a differentialamplifier. The reference voltage is established using a load havingsubstantially the same impedance as the transducer element. The load maybe a second transducer element (as shown in FIG. 1 of the specificationof U.S. Pat. No. 4,751,418) or it may comprise an RC circuit (as shownin FIG. 2 of that specification).

Although the approach to noise reduction described in the specificationof U.S. Pat. No. 4,751,418 may be suitable for adoption with audiofrequency microphones (note that reference is made in that specificationto the use of that invention in the detection of heartbeat sounds), itdoes not confer immunity against noise pick up in the transducerelements themselves, as these have an unbalanced configuration.

DISCLOSURE OF THE PRESENT INVENTION

It is an object of the present invention to provide a novel form ofultrasonic transducer which is substantially less susceptible to theeffects of environmental electrical noise than conventional ultrasonictransducers.

This objective of the present invention is achieved by using a pair oftransducer elements in the transducer, each sandwiched betweenrespective front and back electrodes in the same manner as in the dualback-face electrode transducer. However, instead of having theelectrodes on the front face floating (as in the conventionaltransducer), each front face electrode is earthed. In addition, insteadof having a single input and output for electrical signals supplied toand generated by the transducer, the transducer is provided with a pairof input/output terminals, each of which is connected to a respectiveback electrode of a transducer element.

When the transducer of the present invention is used to generateultrasonic signals, the activating electrical pulse is converted into apair of pulses of opposite polarity. These two pulses are appliedsimultaneously, via the input/output terminals, to their respective backelectrodes of the transducer elements. And when the transducer of thepresent invention is used in the receive mode, the ultrasonic signalincident upon the front face electrodes produces a pair of electricalsignals of opposite polarity at the back face electrodes. When theseelectrical signals are added in a differential amplifier, any commonmode (noise) signals will be cancelled out. Hence the transducer of thepresent invention, when operating in both its transmit mode and itsreceive mode, is "balanced".

Thus, according to the present invention, there is provided anultrasonic transducer comprising

(a) at least one pair of transducer elements, said or each pair oftransducer elements consisting of a first transducer element and asecond transducer element, each transducer element being sandwichedbetween a respective front electrode and a respective back electrode,said or each first transducer element being polarised positive at itsfront electrode and negative at its back electrode, said or each secondtransducer element being polarised negative at its front electrode andpositive at its back electrode; and

(b) first and second input/output terminals; said first input/outputterminal being connected to the back electrode of said or each firsttransducer element, said second input/output terminal being connected tothe back electrode of said or each second transducer element; each ofsaid front electrodes being connected to an earth connection point.

In addition, the transducer of the present invention includes means forconverting an input electrical pulse into a pair of pulses of oppositepolarity which are each connected to a respective one of saidinput/output terminals, and also means for differentially summing theelectrical signals generated at the input/output terminals when anultrasonic signal (pressure wave) is incident upon the front electrodesof the transducer.

Preferably, the conversion of an input signal into a pair of pulses andthe differential summing of generated signals is effected using atransformer which matches the impedance of the transducer element andcable to the impedance of (i) the amplifier which supplies the inputsignal, and (ii) the amplifier to which the electrical signals generatedby the transducer are connected.

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the essential features of a conventionalsingle element ultrasonic transducer.

FIG. 2 is a diagram (similar to FIG. 1) of a conventional dual back-faceelectrode ultrasonic transducer.

FIG. 3 is a diagram showing an ultrasonic transducer constructed inaccordance with the present invention, arranged to operate in both thetransmit and receive mode.

FIG. 4a is a diagram similar to FIG. 3, but with a transformer includedin the transmit and receive arrangement connected to the transducer,which may be used when the transducer impedance is sufficiently low.

FIG. 4b is a preferred alternative to the arrangement of FIG. 4a, whichmay be used when the transducer impedance is high.

FIG. 5 is a partly perspective, partly schematic, illustration of atransducer of the present invention having two rectangular transducerelements, mounted side by side.

FIGS. 6 and 7 illustrate transducers with different transducer elementand electrode shapes, constructed in accordance with the presentinvention.

FIG. 8 shows a disc transducer constructed in accordance with thepresent invention, having eight elements, connected as four pairs ofelements.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

The conventional ultrasonic transducer shown in FIG. 1 has a singledisc-like transducer element 10 mounted between, and in intimate contactwith, a front electrode 11 and a back electrode 12. Although referred toas "disc-like", it will be appreciated that the transducer element mayhave any required peripheral shape, and that the term "disc-like" doesnot imply, in the context of this specification, a limitation to acircular shape. The back electrode is connected to an input/outputterminal 13 by a screened lead 14. The front electrode 11 is connectedto the outer conductor 15 of the screened lead, and thus is alsoearthed. The piezo-electric transducer element 10 is polarised positiveon its front face (which is in intimate contact with the planar frontelectrode 11) and negative at its back face (which is in contact withthe planar back electrode 12).

To produce an ultrasonic signal using the transducer of FIG. 1, anelectrical signal is applied to the input/output terminal 13 through anamplifier 16. A positive pulse at terminal 13 (that is, a positive pulseapplied to the back electrode 12) gives a negative pressure wave in themedium adjacent to the outer surface of the front electrode 11, and viceversa.

When operating in the receive mode, when a positive pressure is appliedto the front electrode 11, a negative-going electrical pulse isgenerated at the input/output terminal 13. The signals generated at theterminal 13 upon receipt of ultrasonic energy at the front electrode 11are amplified by a receiver amplifier 17.

The ultrasonic transducer illustrated in FIG. 2 is also known in thisart. It was introduced to overcome problems encountered in attachingconnectors to the electrode of the front (outer) surface of thetransducer. It is described in, for example, the paper by R. W. Martin,F. E. Silver and A. H. Proctor entitled "Back face only electricalconnections of thickness mode piezo-electric transducers", which waspublished in the IEEE Transactions on Ultrasonics, Ferroelectrics andFrequency Control, Volume UFFC-33, No 6, pages 778 to 781, 1986. Thisdual back-face electrode transducer comprises two piezo-electrictransducer elements 20 and 21, each sandwiched between a respectivefront electrode 22 and a respective back electrode 23. The backelectrode 23 of the transducer element 20 is connected to theinput/output terminal 13 of the transducer by a screened lead 14. Thefront electrodes 22 are connected to each other. The back electrode 23of the transducer element 21 is connected to the earthed shielding orouter conductor 15 of the screened lead. The transducer element 20 ispolarised with its front face positive. The transducer element 21 ispolarised with its front face negative. The transmitter amplifier 16 andthe receiver amplifier 17 operate in the same manner as theircounterparts in FIG. 1.

Thus, when operating in the transmit mode, an electrical pulse which ispositive on the front face of the positively poled transducer element 20produces a positive pressure wave. Also, because the electrodes 22 areconnected together and the two transducer elements are in series, thesame electrical pulse will produce a positive pressure wave from thefront face of the element 21. For a more detailed commentary on theoperation of the transducer of FIG. 2, reference should be made to theaforementioned paper by R. W. Martin, F. E. Silver and A. H. Proctor.

As noted above, ultrasonic transducers of the type illustrated in FIGS.1 and 2 are sensitive to environmental electrical noise. They areconnected in an unbalanced manner and require good earthing of theearthed electrode, and extensive shielding of the connections to thenon-earthed electrode and of the contents of the transducer housing, toensure a good signal to noise ratio, especially when the transducers areoperating in the receive mode.

The embodiment of the present invention which is illustrated in FIG. 3comprises a first transducer element 30 and a second transducer elements31, each having an associated front electrode 32 and back electrode 33.The transducer elements 30 and 31 are each disc-like (but notnecessarily circular) elements of piezo-electric material of the typecommonly used in conventional ultrasonic transducers. The elements 30and 31 are of uniform thickness, with planar front and back faces, andalthough the shape of the element faces is not critical (several usefulshapes are shown in the drawings), the area of the front face of thetransducer element 30 should be essentially equal to the area of thefront face of the transducer element 31. The elements 30 and 31 willalways be mounted closely adjacent to each other.

Each of the front electrodes 32 and the back electrodes 33 are inintimate contact with, and cover the entire front or back face of, theirassociated transducer element 30 and 31, in accordance with conventionalpractice.

The first transducer element 30 is polarised with its front facepositive and its back face negative. The second transducer element 31 ispolarised with its front face negative and its back face positive.

Thus the basic arrangement of transducer elements and front and backelectrodes of a two-element transducer constructed in accordance withthe present invention is essentially the same as that of a conventionaldual back-face electrode ultrasonic transducer. However, the way inwhich the electrodes 32 and 33 are connected, and thus the way in whichelectrical signals are applied to (and received from) the backelectrodes 33, is different from the prior art arrangements.

As shown in FIG. 3, the transducer of the present invention has a pairof input/output terminals 34 and 35. The first input/output terminal 34is connected via lead 36 to the back electrode 33 of the firsttransducer element 30. The second input/output terminal 35 is connectedto the back electrode 33 of the second transducer element 31. Theconnections 36 and 37 between the input/output terminals 34 and 35 andtheir associated back electrodes 33 are formed by a twin shielded cable(an earthed screen or outer conductor 38 is shown in FIG. 3). The frontelectrodes 32 are each connected to earth via the outer conductor 38 ofthis screened lead.

The two outputs of a differential pulse generator 39 are connected tothe input/output terminals 34 and 35. The differential pulse generatoris a known device. It receives an activating pulse for the transducer atits input 39A and produces a pair of pulses of opposite polarity at itsoutputs. When the opposite polarity pulses (which are of equalamplitude) are applied to the back electrodes 33, each of the transducerelements 30 and 31 is excited to generate a pressure wave of the samepolarity for each transducer element. In this respect, the transducer ofFIG. 3 operates effectively as a single element ultrasonic transducer.The transducer of FIG. 3 is thus a balanced system.

When operating in the receive mode, the transducer elements 30 and 31generate electrical signals of opposite polarity at their backelectrodes 33 upon receipt of a pressure wave at their front electrodes32. Using conventional techniques, the signals generated at the backelectrodes 33 are applied to the inputs of a differential receiver 40,and not to the outputs of the differential pulse generator 39. Thedifferential receiver 40 adds the differential received pulses toproduce a single output signal at its output 40A, and rejects commonmode signals.

Thus the ultrasonic transducer shown in FIG. 3 has a significantlyhigher signal to noise ratio in the receive mode than the conventionaldual back-face electrode transducer.

FIG. 4a shows a transducer of the same construction as the transducer ofFIG. 3, but with a transformer arrangement for supplying input signalsof opposite polarity to the input/output terminals 34 and 35, and foradding the differentially generated electrical signals at the terminals34 and 35 when operating in the receive mode. The transformer 43converts a single positive electrical pulse into a pair of equal pulsesof opposite polarity at the terminals 34 and 35. It also adds thedifferentially generated pulses obtained when the transducer isoperating in its receive mode while common mode signals cancel eachother. This configuration provides optimum signal to noise ratio andsignal matching for relatively large transducers.

Large transducers have relatively low impedances which closely resemblethe cable impedance. Those skilled in this art will be aware of theimportance of matching the transmission and receiving system to thecable and transducer element to optimise the performance of thetransducer.

The arrangement shown in FIG. 4a can be used for such tuning, byensuring that the transformer 43 is chosen to match the output impedanceof the amplifier 39 and the input impedance of the amplifier 40 to theimpedance of the cable 38 and assembly of transducer elements 30.

Small transducer elements, such as those used in transducer arrays, havea higher electrical impedance, whereas the typical cable has a lowimpedance. If these two circuit components are not matched, thesensitivity of the transducer arrangement is reduced, in some cases byup to 20 db. Thus the cable and transducer elements should be tuned tominimise the mismatch. The arrangement shown in FIG. 4b can be used forsuch tuning, by ensuring that the transformer 43 is positioned as closeto the transducer element assembly as possible and is chosen to matchthe impedance of the cable 38.

In a linear array of ultrasonic transducer elements constructed inaccordance with the present invention, each transducer element of thearray was provided with a transformer as shown in FIG. 4b. Transducerelements were made from PZT5 material, with a length of 14 mm and awidth of 1 mm. Each transformer comprised a ferrite core having a volumeof 14 cubic mm and was wound to match the impedance of the transducerelements with the associated cable. This linear array has producedsubstantially better signal to noise performance than any other lineararray of ultrasound transducers used by the present inventor.

A further improvement in the signal to noise performance can be achievedby matching the low signal cable impedance, using a step up transformerat the input to each receiver. A four to one step up is a realisticpractical requirement for such a transformer.

The present inventor has also found that the operation of ultrasonicechoscopy equipment having conventional ultrasound transducerarrangements can be improved by the use of a matching transformer in themanner shown in FIGS. 4a and 4b (but with a conventional ultrasonictransducer in place of the element constructed in accordance with thepresent invention). This modification of a conventional ultrasonictransducer constitutes a further aspect of the present invention.

FIGS. 5, 6 and 7 illustrate examples of different transducer elementshapes that may be used in a transducer of the present invention havingtwo transducer elements. The useful transducer element shapes are notlimited to those depicted in FIGS. 5, 6 and 7.

FIG. 8 shows one example of the way in which a transducer having morethan two elements can be constructed in accordance with the presentinvention. In the FIG. 8 embodiment, the disc-like transducer elementhas been divided into eight sub-elements of equal area. The sub-elements82, 84, 86 and 88 are poled positively and have their back electrodesconnected together and to one input/output terminal of the transducer.The sub-elements 81, 83, 85 and 87 are poled negatively and have theirback electrodes connected together and to the other input/outputterminal of the transducer. The front electrodes of all of thesub-elements are connected to earth, via the screening conductor 38.

To construct a transducer of the form illustrated in FIGS. 6 and 8, theindividual transducer elements are best formed using a single crystal,with the poling of the individual transducer elements effected using thetechnique described in the aforementioned paper by R. W. Martin, F. E.Silver and A. H. Proctor, or a modification of that technique.

The embodiments of the present invention described above all feature asingle ultrasonic electroacoustical transducer. It will be appreciatedthat the present inventive concept of balanced transducer elements andelectronic components can be applied in the implementation of arraytransducers, including annular, linear, curved linear, and phased arraytransducers as commonly used in this art.

It will also be appreciated that although a number of embodiments of theultrasonic transducer of the present invention have been illustrated anddescribed in this specification, the present invention is not limited tothose embodiments. Variations to and modifications of the illustratedembodiments are possible without departing from the present inventiveconcept.

I claim:
 1. An ultrasonic transducer comprising:(a) at least one pair oftransducer elements, said or each pair of transducer elements consistingof a first transducer element and a second transducer element, eachtransducer element being sandwiched between a respective front electrodeand a respective back electrode, said or each first transducer elementbeing polarised positive at its front electrode and negative at its backelectrode, said or each second transducer element being polarisednegative at its front electrode and positive at its back electrode; (b)first and second input/output terminals; said first input/outputterminal being connected to the back electrode of said or each firsttransducer element, said second input/output terminal being connected tothe back electrode of said or each second transducer element; each ofsaid front electrodes being connected to an earth connection point; and(c) a differential pulse generator having an input, and a pair ofoutputs and a differential summing device having a pair of inputs and anoutput, said first input/output terminal being connected to one of theoutputs of the differential pulse generator and also to one of theinputs of the differential summing device, said second input/outputterminal being connected to the other of the outputs of the differentialpulse generator and also to the other of the inputs of the differentialsumming device;whereby (i) a voltage pulse applied to the input of thedifferential pulse generator produces a pair of pulses of oppositepolarity at, respectively, the two outputs of the differential pulsegenerator, each of said pulses of opposite polarity being applied to arespective one of the back electrodes, and (ii) an ultrasound pressurewave incident upon the front electrodes generates a pair of oppositepolarity pulses at the back electrodes, each of said pulses at the backelectrodes being connected to a respective one of the inputs of thedifferential summing device, to generate a single pulse at the output ofthe differential summing device.
 2. An ultrasonic transducercomprising:(a) at least one pair of transducer elements, said or eachpair of transducer elements consisting of a first transducer element anda second transducer element, each transducer element being sandwichedbetween a respective front electrode and a respective back electrode,said or each first transducer element being polarised positive at itsfront electrode and negative at its back electrode, said or each secondtransducer element being polarised negative at its front electrode andpositive at its back electrode; (b) first and second input/outputterminals; said first input/output terminal being connected to the backelectrode of said or each first transducer element, said secondinput/output terminal being connected to the back electrode of said oreach second transducer element; each of said front electrodes beingconnected to an earth connection point; (c) a first operationalamplifier, having an input and an output; (d) a second operationalamplifier having an input and an output; and (e) a transformer having asingle primary winding and first and second secondary windings; saidsecondary windings being connected in series and being earthed at theirpoint of connection to each other; the output of said first operationalamplifier being connected to the end of said first secondary windingwhich is not connected to earth; the input of said second operationalamplifier being connected to the end of said second secondary windingwhich is not connected to earth; said first input/output terminal beingconnected to one end of said primary winding; and said secondinput/output terminal being connected to the other end of said primarywinding;whereby (i) a positive pulse at the output of the firstoperational amplifier generates a pair of opposite polarity pulses atthe ends of said primary winding, each of said pulses of oppositepolarity being applied to a respective one of said back electrodes, and(ii) an ultrasound pressure wave received at said front electrodesgenerates a pair of opposite polarity pulses at said back electrodes,each of such pulses at said back electrodes being connected to arespective end of said primary winding and causing a single pulse,proportional to the differential sum of the pulses at said backelectrodes, to be generated by said second secondary winding and appliedto the input to said second operational amplifier.
 3. An ultrasonictransducer as defined in claim 2, in which said transformer is wound tomatch the input and output impedance of said first and secondoperational amplifiers to the electrical impedance of the transducerelements.
 4. An ultrasonic transducer as defined in claim 3, including arespective step up transformer associated with the input and output ofsaid first and second operational amplifiers, for improving the matchingof the input and output impedances of said operational amplifiers to theelectrical impedance of the transducer elements.